Implemented Sheet.visibility for xlsx files.
Ignoe anchors ($) in cell references
Revert cell_name_to_rowx_colx.  Remove anchors in do_row only.
github issue 64: need to skip meaningless chunk of 4 zero bytes between two otherwise-valid BIFF records; was being treated as a DIMENSIONS record and then crashing.
Don't use str.replace on cell_name
Merge pull request #65 from Bklyn/master

Ignore anchors ($) in cell references